'Your private emails are being read by third-party Gmail app developers, an investigation into data privacy has revealed.

Developers behind a number of popular online services designed to work with Gmail trawl through private messages sent and received from your email address, it claims.

It is common practice for some of these third-party app creators to instruct employees to read personal emails.

One app, which is designed to help users manage their Gmail inbox, lets employees read 'thousands' of emails, the Wall Street Journal investigation found.

According to experts, this 'dirty secret' is now common practice among some firms.

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The revelation comes just a few months after it was revealed political data firm Cambridge Analytica had siphoned private data from third-party apps on Facebook.

According to the investigation into Gmail, the hugely-successful Google email client allows third-party developers to scan the inbox of anyone who installs their app.

These apps can provide additional functionality to the Gmail inbox, like the ability to compare prices from different online retailers, or quickly unsubscribe from any marketing emails sent to your address.

The Wall Street Journal report was based on the testimonies of more than two dozen employees of companies who create services around Gmail – the most popular email service in the world, with 1.2 active monthly users.

One company involved in this practice is New York-based firm Return Path, which helps marketers drive revenue through email.